1. Field Of The Invention
The present invention relates to optical scanners and more particularly to controlling the scanning rate and illumination intensity in such a scanner as a function of the velocity of a moving web being scanned.
2. Description Relative To The Prior Art
It is well known in the prior art to detect the presence of defects, splices or other irregularities or discontinuities in a moving web by illuminating a transverse region of the web and scanning a region within the illuminated region. Such scanning may be performed by an electronic camera employing either a linear array of photodetectors that view corresponding areas of the illuminated region and that are read out serially, or by means of a single photodetector combined with an optical system that sweeps the viewing area of that detector along the illuminated region.
To detect relatively small or subtle irregularities in a rapidly moving web, the illumination must be of relatively high intensity. For example, U.S. Pat. No. 4,752,897, issued on June 21, 1988 in the names of Leon Zoeller et al. discloses such an apparatus capable of scanning a web at extremely high speeds, requiring handling of the resulting digital signals at data rates of the order of tens of megabytes per second. This requires quite brilliant illumination because of the extremely short duration during which each individual area of the web or the like is accessed. Correlatively, the illumination must be uniform across the width of the web so that variations in the intensity of the light received by the photodetector are attributable to variations in the web and not to variations in its illumination. In accordance with the invention disclosed in the above-identified commonly owned U.S. Pat. Applications, Ser. Nos. 289,875 and 289,872, the latter requirement is achieved by illumination means comprising a single elongate incandescent filament supported only by its ends.
The foregoing Zoeller et al. U.S. Pat. also discloses the concept of maintaining equal spacing of the scan regions by using the clock signals that control the scanner to adjust the web speed so that the web moves past the scanner at the exact speed required to achieve that equality, but does not provide means for maintaining the equality of the scan region spacing if the speed of the web varies significantly, as is often dictated by other operations being performed on the web. Also, no means are disclosed for controlling illumination of the web scanning region as a function of web speed so that such speed changes do not influence the output of the camera.